Dual-Band Biosensing With van der Waals Assisted Optical Metasurfaces

In this paper, we theoretically investigate a van der Waals (vdW) heterostructure-based optical metasurface for dual-band optical biosensing capable of attaining refractive index (RI) sensing in the visible and vibrational fingerprint detection of the same analyte in the mid-infrared (mid-IR) region simultaneously. The proposed structure consists of two gratings – Calcium Fluoride (CaF ₂ ) grating to produce guided-mode resonance in visible and silver (Ag) grating coupled with graphene/ hexagonal boron nitride (hBN) heterostructure to induce hybrid plasmon phonon mode in mid-IR that can be used for surface-enhanced infrared absorption (SEIRA) spectroscopy. The evanescent wave in CaF ₂ gratings gives rise to RI sensing at $\sim ~689$ nm. The presence of graphene adds tunability and thus allows identification of multiple vibrational fingerprints of organic molecules, nitroethane and nitrobenzene in mid-IR, with a sensitivity of 360.6 nm/RIU and 313.4 nm/RIU, respectively, in the visible region. The fingerprint signal gets enhanced at 6.43 $\mu \text{m}$ (nitroethane) and 6.56 $\mu \text{m}$ (nitrobenzene) by varying the Fermi potential of the graphene layer above 0.3 eV. The proposed sensor has the potential to meet the requirement of next-generation medical applications such as disease diagnostics, health monitoring, and food/environment monitoring as the sensor enables label-free, sensitive, rapid detection of organic molecules.